This invention relates to a polarization-independent optical wavelength filter having a simplified two-stage structure.
Optical wavelength filters have been developed for use in wavelength-division-multiplexed lightwave communication systems, to select a desired signal channel having a certain wavelength while rejecting other channels of different wavelengths. One conventional filter design employs a TE-TM mode converter disposed between crossed polarizers. This type of filter combines sharp wavelength selectivity with low loss, but it restricts the optical input signal to a specific state of polarization, a restriction which is extremely inconvenient in lightwave communications.
To overcome this restriction, various polarization-independent designs have been proposed. One design, described in Japanese Patent Kokai Publication 84621/1990, divides the input signal into two halves which are red through two parallel polarization-dependent wavelength filters, one transmitting a specific wavelength of the TE component, the other transmitting the same wavelength of the TM component. The emerging signals are recombined to obtain the output signal. Aside from requiring two wavelength Filters, this design has the disadvantage of losing half of the desired signal.
Another design, described in the same source employs one polarization-dependent wavelength filter and rotates the plane of polarization of the input signal through a variable angle, the angle being varied cyclically at a rate higher than the signaling care. This design also has the disadvantage of losing half of the desired signal.
Yet another design, disclosed by the present inventors in Japanese Patent Kokai Publication 169603/1991, uses a first polarization splitter to divide the input signal into two linearly-polarized components, which are rotated so that their planes of polarization coincide. These components are coupled to opposite ends of a polarization-dependent filter that separates the desired wavelength. The emerging signals are again rotated, then coupled to a second polarization splitter that recombines them into an output signal. This design avoids the large loss associated with the two preceding designs, but is structurally complex.